enough to send an electrical charge through the wires."

"So don't sit on the handle," Rick joked.

"And don't kick it," Scotty added.

Zircon connected the wires to a pair of terminals on top of the
detonator, then looked at his watch. "Plenty of time. We might as well
take it easy. Anyone hungry?"

No one was. It was too soon after breakfast. Instead, Rick took the
opportunity to ask questions.

"I can understand the general principle of what we're doing, but can you
tell us exactly what happens?"

"Sure. When the dynamite charge goes off, it sends shock waves through
the earth in all directions. Whenever a shock wave strikes something of
different density, its direction and velocity change. For instance, if
there is denser rock a few hundred feet down, that will cause a change
of both velocity and direction. With me so far?"

"I think so," Scotty said. "The denser the stuff the wave strikes, the
faster it moves. Like sound waves. I mean, sound moves faster in water
than in air, and faster in a steel rail than in water. Is it the same?"

"Just about," Zircon agreed. "The shock waves radiate away from us,
through the earth, and eventually reach the recorders on the other side
of the mountain. You can see what happens, I think. Waves will arrive at
different times, depending on the path they took and the kind of
material they went through."

Rick nodded. "So if there's molten rock, or magma somewhere in the way,
the shock wave that goes through it will slow down and arrive at the
recorder later?"

"That's it. The tracings we get can be analyzed to give us a kind of
cross-sectional look at the mountain. You see, we know how fast the
waves travel through different kinds of earth structure. Also, we will
know the point of the explosion and the location of the recorder for
each shot. Which reminds me. We'd better get out the equipment and
locate ourselves precisely."

"How?" Rick asked. "What will we use for landmarks?"

"The top of the mountain, for one, and if you'll look carefully to a
point slightly south of east between those two banana palms, you'll see
the top of the control tower at the airport."

Rick shook his head. "Good thing you're with us. I completely forgot to
watch for landmarks."

"That was the first thing I had in mind in looking for a spot," Zircon
told him.

The transit gave a precise angle between the two landmarks. Zircon drew
a line on the map connecting the southern tip of the m